This invention relates to a cordierite body which has a low coefficient of thermal expansion (CTE). The invention relates also to a method of fabricating the body which makes use of a composition of raw materials in which at least a portion of the raw materials is provided as magnesium aluminate spinel which will be referred to as spinel. The raw materials are absent of clay and absent of talc. More particularly, the body is formed by extrusion. Still more particularly, the extruded body has a honeycomb structure. A cordierite body having a honeycomb structure is especially suited for but not limited to use as a substrate for a catalytic converter for automobiles.
Extruded cordierite honeycombs are manufactured for use as substrates to support catalytically active components for catalytic converters on automobiles. Use of cordierite is favorable in this application because of its good thermal shock resistance. The thermal shock resistance (TSR) is inversely proportional to the coefficient of thermal expansion (CTE). That is, honeycombs with a low thermal expansion have good thermal shock resistance and can survive the wide temperature fluctuations that are encountered in the application.
Up to this time cordierite has been made typically with starting raw materials of clay, talc and alumina. U.S. Pat. No. 3,885,977 relates to extruded honeycombed monolithic fired ceramics made with these raw materials. In this patent it is stated that the orientation of the cordierite crystals in the fired body as measured by the I ratio, comes about because cordierite forms on oriented clay particles which are aligned by the extrusion process resulting in the desired low CTE. Therefore it was believed that the clay was necessary to form cordierite.
U.S. Pat. No. 4,280,845 relates to cordierite ceramic fabricated from a mixture of magnesia material, alumina, and silica raw materials using clay and talc predominately. The magnesia has a particle size of 5-150 microns. Although there are some examples given of using ternary end member oxides, that is magnesium oxide, aluminum oxide, and silica without clay or talc, there is no teaching of using magnesium aluminate spinel as a raw material.
In U.S. Pat. No. 4,434,117, it has been further suggested that the use of a platy talc without clay is a ncessary and sufficient condition in order to achieve a cordierite body with a low coefficient of thermal expansion when this platy talc is combined with other non-clay components to obtain the cordierite composition.